In an electric double-layer capacitor and a secondary battery, water-soluble electrolyte and nonaqueous electrolyte with an organic solvent are used. The nonaqueous electrolyte has the advantages of chargeable at high-voltage and raising energy density. On the other hand, the nonaqueous electrolyte has flammability, and thus the safety of the electric double-layer capacitor and the secondary battery has been a significant problem for a long time. For example, for a lithium ion secondary battery having a high capacity, in order to insure the safety, a protective circuit has to be used in combination with the battery so as to prevent overcharge and overdischarge.
In order to solve the problems caused by flammability of the nonaqueous electrolyte, it has been proposed to use a room temperature molten salt, namely the so-called ionic liquid, which is a liquid at room temperature, and has a tiny vapor pressure and is nonflammable. Japanese Patent No. 2981545 discloses a room temperature molten salt comprised of trimethylhexyl ammonium ion as a cation and N(CF3SO2)2— as an anion. The mixture of said molten salt and LiN(CF3SO2)2 can be used as a nonaqueous electrolyte of a lithium ion secondary battery.
Although the room temperature molten salt is generally liquid at room temperature, and it tends to become jelly or to solidify in a supercooled state, over a period of time (for example, one day to several weeks). As a result, it is problematic that the viscosity of the nonaqueous electrolyte will rise and accordingly the internal resistance of the electric double-layer capacitor or the secondary battery will increase.
Specially, in a lithium ion secondary battery, in the case where a room temperature molten salt and a lithium salt are used as a nonaqueous electrolyte, problems such as the molten salt crystallizing out of the electrolyte will occur.
Therefore, in an electric double-layer capacitor, the room temperature molten salt generally may be used in a wide range of temperatures of from −20° C. to 85° C., it has the shortcoming of having a relatively small capacity specially at low temperature.
Furthermore, with the ever-increasing performance of electronic equipment, there is a demand for discharge load characteristics capable of high-rate discharge. However, the secondary battery of the prior arts is unable to meet this demand.